This invention relates generally to mass spectrometers, and more particularly to double focusing mass spectrometers.
Mass spectrometers have earned a respected reputation for their unique ability to identify and quantitate a wide variety of chemical elements and compounds, often present in only trace level concentrations in complex chemical mixtures. Operating in a vacuum chamber, mass spectrometers ionize and fragment sample molecules, and through use of appropriate control and data capture electronics, generate a histogram of fragment molecular weight versus relative abundance of each ionic species present in the sample.
One class of mass spectrometer called a magnetic sector instrument uses a magnetic field at right angles to the ion beam trajectory to separate ions based on their mass-to-charge ratios. Single focusing magnetic sector instruments perform only directional focusing, while double focusing magnetic sector instruments provide both direction and velocity or energy focusing of the ion beam, usually by using an additional electrostatic energy analyzer in tandem with the magnetic sector analyzer, to achieve significantly higher resolution. Traditionally, both single and double focusing mass spectrometers are bulky, typically weighing 100-1500 kg, and thus confined to analytical laboratories due to their large size, weight, high power consumption and need for frequent service by skilled operators. Furthermore, due to their complexity and relatively low unit production rates, they are traditionally hand-made, one at a time, at a relatively high unit cost.
For example, U.S. Pat. No. 3,984,682 to H. Matsuda and U.S. Pat. No. 4,054,796 to M. Naito, show that the size of double focusing mass spectrometers may be significantly reduced by arranging the electric sector energy analyzer within the magnet pole gap. In contrast to tandem electric and magnetic sector analyzers, each of which contributes to the total ion path length, one obvious advantage of superimposing the magnetic and electric fields is a smaller instrument with a shorter ion path length between the ionizer or source and the ion detector. However, this advantage, which reduces the number of ion-molecule collisions, creates new challenges that must be overcome in order to achieve the desired performance. Specifically, it is difficult to generate the required orthogonal electric and magnetic fields within a single small volume without causing serious electric field degradation effects that degrade performance. According to Matsuda and Naito, two cylindrical sector, coaxially-aligned electrodes connected to a voltage source generate a radial electric field used in the energy analyzer portion of a double focusing mass spectrometer. In order to accommodate the small magnet gap axial dimension, these concentric electrodes must have a low axial height-to-separation ratio in order to allow sufficient radial separation to permit ion transmission along the central orbit of the ion beam. The small axial height of these electrodes results in undesirable fringe field effects in the ion path and thus sub-optimal resolution. Further, auxiliary electrodes are added between the upper and lower edges of these cylindrical electrodes to adjust the electric field in the central ion orbit to improve transmission and resolution. While the use of auxiliary electrodes greatly improves the geometry of the electric field in the electric sector, the presence of the cylindrical electrodes ultimately limits the further reduction of the magnet pole gap.
In addition, traditional electromagnets used to generate a 1-2 Tesla magnetic field in a 2 to 10 cm magnet pole gap are prohibitively expensive and bulky compared to rare earth permanent magnets now commonly used in smaller instruments requiring a fixed or non-scanning magnetic field. Furthermore, even with the newest high-energy-product NdFeB magnets, it remains extremely difficult and costly to generate 1-2 Tesla in a magnet pole gap larger than 1 cm.
Further, known mass spectrometers are large in physical size requiring significant installation space, usually in a well-regulated operating environment where temperature, humidity, vibration and other conditions are tightly controlled. Known mass spectrometers use heavy construction materials favoring discrete, usually stainless steel, components including vacuum manifolds, flanges, valves and supporting structural elements. This necessitates low quantity manual construction of each instrument at a relatively high unit cost. Also, known mass spectrometers require high electrical power consumption to run vacuum pumps, heaters, air conditioners, water coolers, electronics and ancillary equipment. Also, the complex design of known mass spectrometers require full-time, specially-skilled operators to use the equipment and perform routine maintenance and repairs, often requiring delicate alignment of internal elements and an inventory of spare parts.
The relatively large physical dimensions of present mass spectrometers require a lower operating pressure than smaller instruments in which ions traverse a shorter path between the ion source and detector. The mean free path length of a molecule in a vacuum system is inversely proportional to pressure and can be approximated by xcex=0.005/P, where xcex is the mean-free path length in centimeters and P is the pressure in Torr. As a general design rule, vacuum pumps are employed that maintain the mean free path length to an order of magnitude longer than the actual ion flight path length. Accordingly, microscale instruments can operate at higher sample gas pressure and require smaller, less expensive vacuum pumps. Since vacuum pumps represent some of the highest cost components in conventional mass spectrometers, a significant cost reduction benefit results from smaller, less expensive vacuum pumps.
Additionally, known double focusing mass spectrometers with larger electrode separations require higher voltages to create the same electric field as in smaller, functionally equivalent instruments in which electrodes are closer together. For example, a voltage source of 1000 volts is required to produce an electric field of 10,000 volts/meter between planar electrodes 10 cm apart, while a voltage source of only 50 volts is required to produce the same electric field between electrodes 5 mm apart.
Still further, known double focusing mass spectrometers have traditionally been constructed primarily of stainless steel housings, bolts, valves, transfer lines and structural supports with an essentially unlimited lifetime. Such construction has generally limited these instruments to use in stationary operating environments, typically laboratories or industrial plants.
In one aspect, a mass sensor is provided. In an exemplary embodiment, the mass sensor includes a magnet assembly and a mass analyzer. The mass analyzer includes a housing having a cavity therein with the housing including a first plate, a second plate, and a center portion positioned between the first and second plates. The center portion includes an outer wall. The mass analyzer also includes an ionizer a double focusing mass spectrometer having superimposed orthogonal magnetic and electric fields, and an ion detector. The ionizer, the double focusing mass spectrometer, and the ion detector are located in the housing cavity. The double focusing mass spectrometer includes an electric sector energy analyzer. The electric sector energy analyzer includes a first element located on an inside surface of the first plate, and a second element located on an inside surface of the second plate. The first and second electric sector energy analyzer elements are substantially concentric and congruent and have a circular arc shape. Each first and second element include a first boundary electrode, a second boundary electrode, and a continuous resistive material extending between the first and second boundary electrodes.
In another aspect, a mass analyzer is provided that in an exemplary embodiment includes a housing having a cavity therein with the housing including a first plate, a second plate, and a center portion positioned between the first and second plates. The center portion includes an outer wall. The mass analyzer also includes an ionizer a double focusing mass spectrometer, and an ion detector. The ionizer, the double focusing mass spectrometer, and the ion detector are located in the housing cavity. The double focusing mass spectrometer includes an electric sector energy analyzer. The electric sector energy analyzer includes a first element located on an inside surface of the first plate, and a second element located on an inside surface of the second plate. The first and second electric sector energy analyzer elements are substantially concentric and congruent and have a circular arc shape. Each first and second element include a first boundary electrode, a second boundary electrode, and a continuous resistive material extending between the first and second boundary electrodes.